The present invention generally relates to AC-to-DC converters and more particularly to passive AC-to-DC converters with voltage boosting capability.
AC-to-DC converters play a significant role in the modern aerospace/military industry. This is particularly true in the area of more electric architecture (MEA) for aircraft and spacecraft. Power quality is a major concern for MEA aircraft because of the large number of electric power systems and equipment installed on the same bus. The power quality of these systems and equipment has stringent requirements to ensure that all power supplies/utilization equipment function together properly.
The term “composite AC-to-DC converter” has been coined to distinguish a converter using two or more conversion methods in parallel. The concept for a composite AC-to-DC converter originated as a further improvement towards smaller size, lower weight, and higher efficiency.
While composite AC-to-DC converters present a large step toward performance improvement they have not incorporated efficient boosting capabilities. They typically provide rectification of a three phase 115-Vac system resulting in a typical output voltage value of 270 Vdc. There are many applications where the output voltage is desired to be much higher for a better performance of a consecutive power conditioning. Typical values used in some power distribution systems are 540 Vdc, +/−270 Vdc and 610 Vdc. That means that it would be desirable for a composite AC-to-DC converter, used in a three phase 115-Vac system, to produce output voltage about two times higher at its rectified output. In other words, it would be desirable to provide voltage boosting capability in a composite AC-to-DC converter.
It would be desirable to achieve such voltage boosting passively while introducing only minimal harmonic distortions to input AC currents. Typically, lower frequency harmonic distortions may be reduced by employing AC-to-DC converters with high pulse configurations. For example, when conversion of three phase AC power is performed with a 24-pulse converter, harmonic distortions of input power may be maintained at a reasonably low level. Of course, a 24-pulse converter must have a higher number of transformer windings than a 12-pulse or 18-pulse converter. Consequently, 24-pulse converters are typically heavier, larger and more expensive than 12-pulse or 18-pulse converters. Such sizes and weights may be evaluated objectively by considering and expression W/VA: where W is DC power in watts; and VA is the rating of a transformer expressed in volt-amperes. For a typical 18-pulse converter with a 1:2 boosting capability, W/VA may be about 0.5.
Additionally, it would be desirable to achieve passive voltage boosting employing an autotransformer, as compared to an active semiconductor-based boosting circuit. In the context of aerospace applications inherent reliability of a passive system as compared to an active system is an important consideration.
Within an autotransformer of a composite AC-to-DC converter, interior winding turn ratios are responsible for its voltage boost factor. However a typical autotransformer's conversion ratio (ACR) will begin to decrease when used for voltage boosting. The main cause of a decreasing ACR in a boost topology can be viewed as the autotransformer winding volts*amperes (VA) sum is going up while the autotransformer output power is remaining constant. A high ACR is desirable in an autotransformer used in an aerospace vehicle because such an autotransformer may be constructed with smaller windings and with less weight than an autotransformer having a low ACR.
As can be seen, there is a need for a passive composite AC-to-DC converter with voltage boosting capability. More particularly, there is a need for such a converter that may produce voltage boosting passively with an autotransformer which may operate with a high ACR and with minimal harmonic distortion of input voltage.